Reveal the Cause of Downhole Restrictions by Means of Knowledge Engineering

Abstract

Stuck pipe is an example of an unwanted problem that causes high NPT and seemsto occur again and again. As 50 % of all stuck pipe incidents occur during tripping,increased knowledge about how to recognize progressing downhole problems areneeded. Early warnings of stuck pipe are HKL restrictions, and early warnings ofHKL restrictions are symptoms. If the symptoms seen at surface can be relatedto a HKL restriction type, preventing actions can be initiated at an early stage,and stuck pipe can be avoided.Four main causes of HKL restrictions have been analysed and related to symptomsthat can be recognized at the surface. The main causes are accumulating cuttings,mechanically unstable wellbore, chemically unstable wellbore and local dogleg. Bymeans of the Knowledge Model, relationships between the symptoms and the maincauses were determined. The strength of the relation between each symptom andthe restriction cause was found and proved diversity among the main symptomsof each restriction cause.21 cases of HKL restrictions were found from drilling reports and RTDD obtainedfrom Statoil. Symptoms interpreted from the drilling reports were used as inputfor the Knowledge Model which revealed the main cause of the restriction. Themodel proved that 14 of the 21 restriction cases were caused by the error ReactiveFormation.The HKL signature of the 21 cases was interpreted and related to the restrictioncauses. The interpretation showed that the signature of cuttings accumulating canbe separated from the signatures of mechanically instability.Five restriction events were analysed using the Knowledge Model. The main causeof the restriction events were found proving the Knowledge Models ability to de-termine the restriction cause of downhole problems. The analysis proved thatReactive Formation was the most frequent error to cause HKL restrictions, withexplanatory strength of 48 % as the highest in average.